Inkjet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand inkjet, individual droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the ejection of ink droplets in drop-on-demand printing include thermal bubble formation (thermal inkjet (TIJ)) and piezoelectric transducers. In another process known as continuous inkjet (CIJ), a continuous stream of droplets is generated and expelled in an image-wise manner onto the surface of the image-recording element while non-imaged droplets are deflected, caught, and recycled to an ink sump. Inkjet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
Ink compositions containing colorants used in inkjet printers can be classified as either pigment-based, in which the colorant exists as pigment particles suspended in the ink composition, or as dye-based, in which the colorant exists as a fully solvated dye species that consists of one or more dye molecules. Pigments are highly desirable since they are far more resistant to fading than dyes. However, pigment inks can have inferior durability after printing, especially under conditions where abrasive forces have been applied to the printed image and especially at short time intervals from immediately after printing to several minutes while the inks are drying.
Pigment-based inks must be ejected from a printhead, reliably for numerous individual firing events during the lifetime of a printer. As an example, a typical inkjet nozzle may be required to fire in excess of 5×107, and up to as many as 1×109, individual firing events without malfunctioning or ceasing to fire altogether. This includes situations where the printhead is left idle or uncapped for long periods of time and then is actuated again to eject ink. In some instances the idle printhead nozzles can partially clog or crust with ink components thereby degrading the ability of the printhead to eject properly. For example, the ink can be misdirected from the partially clogged nozzles or the drop velocity can be greatly diminished. In some instances, the nozzle will become permanently clogged and in other instances a lengthy and costly maintenance operation may be required to recover the nozzle back to a usable state of operation. This phenomena is known in the art of inkjet printing as latency or de-cap.
One measurement of latency performance involves measuring the initial drop velocity of an ink that is firing at a useful steady state condition, followed by the measurement of the drop velocity after some time interval during which the nozzle is idle, uncapped, and exposed to ambient conditions. The drop velocity after the latency interval can vary depending on how many ejection pulses are applied to recover the nozzle back to a useful drop velocity. An ink having good latency performance would exhibit a useful drop velocity after long de-cap intervals with a minimum number of ejection pulses. Therefore, a longer latency is highly desirable as the ink can reside in the idle printhead for a longer time without adversely affecting the ink ejection performance.
Pigment-based inks formulated with polymeric dispersants and binders can be difficult to jet through inkjet printheads having small nozzle diameters especially by the thermal inkjet printing process. This is especially true of pigment-based inks, which are formulated with humectants or penetrants that lower dynamic surface tension. In recent years, thermal inkjet printers have moved to higher jetting frequencies to provide faster printing speeds. Thermal inkjet printers are now capable of printing at jetting frequencies in excess of 10 kHz and the ability for higher velocity firings is a highly desirable feature. However, this high frequency firing often comes at the cost of variability in the drop velocity, which leads to poor image quality in the final printed image.
Polyurethane binders have been used as durability enhancing additives in dye-based and pigment-based inkjet inks. U.S. Pat. No. 6,136,890 discloses a pigment-based inkjet ink wherein the pigment particles are stabilized by a polyurethane dispersant. US Publication No. 2004/0242726 discloses a pigment dispersed by a cross-linking step between a resin having a urethane bond and a second water-soluble polymer. US Publication No. 2004/0229976 discloses polyurethane/polyurea resins for pigmented inks where the weight fraction of a polyurethane urea part is at most 2.0 wt % to the urethane resin.
Although polyurethanes are known for their excellent durability, they also have a number of drawbacks. For example, not all polyurethane polymers are conducive to jetting from a thermal inkjet head. In particular, water-dispersible polyurethane particles, such as those disclosed in U.S. Pat. Nos. 6,533,408; 6,268,101; Statutory Invention Registration No. U.S. H2113H; and US Publication Numbers 2004/0130608 and 2004/0229976 are particularly difficult to jet from a thermal inkjet printhead at high firing frequencies. The molecular weight of the polyurethane binder plays an important role in the ink performance and durability of the resulting printed images. For example, molecular weights below about 8,000 generally do not provide highly durable images. On the other hand, molecular weights above about 50,000, and especially above 150,000, can raise the viscosity of the ink, which can be detrimental to firing performance from a thermal inkjet printhead. The acid number of the polyurethane also creates limitations for use in an inkjet printing system. If the acid number of the polyurethane is too high the resulting abrasion resistance of the image can become degraded, especially under conditions of high temperature and high humidity. If the acid number of the polyurethane is too low a substantial amount of particulate polymer will exist and jetting can become degraded.
It is also known in the art of pigment-based inkjet inks to combine a polyurethane with a second polymer, such as an acrylic polymer or polyester. U.S. Pat. No. 6,794,425 discloses a mixture of a hydrophilic polyurethane and a hydrophobic polymer where the molecular weights of polymers are specified. US Publication Number 2003/0166742 discloses the combination of a polyurethane and a second polymer where the acid numbers of the polymers are specified. However, such ink formulations can exhibit short latency times in those inkjet printheads. The latency of such ink formulations can be further degraded by the choice of water-soluble organic solvents used to provide humectancy or penetration into the inkjet receiver.